Researchers at the Cleveland Clinic have developed a protein that makes laboratory mice highly resistant to breast cancer:

Cancer vaccines, unlike traditional vaccines, are generally designed to arm the immune system with the tools to fight a certain existing disease or cancer-causing virus, rather than directly prevent cancer from developing. But researchers have found a protein manufactured in cancerous breast cells that primes the immune system to attack tumor cells themselves and prevent the growth of tumors altogether.

In mice that had been pre-engineered to develop breast cancer, an injection of the protein stopped the cancer from ever forming. Healthy lactating cells produce the protein as well, and as such it could one day provide a vaccine that prevents breast cancer in non-lactating women.